As of 2.1, the build server must have PostSharp installed as well as any developers who need to build the projects. If you don't then the build will fail and they will probably not behappy with you. PostSharp is part of the build process which is why it's needed on the build server.

You cannot have PostSharp ignored. If the other devs are able to reference PostSharp.dll to satisfy the types and it actually built then they would have invalid output because it doesn't have the aspects applied in the final assemblies.

I recommend installing PostSharp into the source repository (http://doc.sharpcrafters.com/postsharp-2.1/Content.aspx/PostSharp-2.1.chm/html/41129237-a5d4-4acf-aef4-c46759ccfb4b.htm) to make it easier on everyone.